Method of improving the audibility of sound from a loudspeaker located close to an ear

ABSTRACT

A method and apparatus for improving the audibility of sound from a loudspeaker located close to an ear, comprises the steps of detecting ambient acoustic noise arriving from other sound sources using a transducer that provides corresponding ambient sound signals, inverting the polarity of said ambient sound signals and combining them with the signals being fed to the loudspeaker to reduce the audibility of said ambient acoustic noise, and passing said ambient sound signals through a filter having a predetermined average transfer function that compensates for the spectral modification of sounds travelling from the loudspeaker to the ear caused by the proximity of the ear of a listener in use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of improving the audibility ofsound from a loudspeaker located close to an ear. It relatesparticularly, though not exclusively, to an improved method of providingacoustic noise reduction in telephones and headphones, such that thelistener can perceive the transmitted sounds more effectively in a noisyenvironment. It also relates to apparatus for use in the method.

2. Discussion of the Related Art

The principle of acoustic noise cancellation is well known. Theunwanted, incoming noise signal is received by a transducer such as amicrophone; the resultant signal is inverted and broadcast via aloudspeaker, or similar actuator, so as to combine with the originalacoustic noise signal at a point set slightly apart from the pickupmicrophone. (The paper “The active control of sound”, S J Elliott and PA Nelson, Electronics and Communication Engineering Journal, August1990, pp. 127–136, contains a useful reference listing.) It has beenemployed with varying success over many years for a variety ofapplications. For example, it has been speculated that large-scale noisecancellation might be used to quiet industrial office environments, butwith little success. The problem is that whilst it is theoreticallypossible to cancel noise at a single point in space, the cancellationdoes not persist at other points in space because the magnitude andphase of the unwanted signals cannot be matched. Indeed, thecancellation signal itself contributes to the overall noise level awayfrom the cancellation spot.

Known noise-reduction systems can be divided into two categories: (a)systems based on loudspeakers located far from the ear, and (b) systemsbased on loudspeakers located close to or adjacent the ear (includingheadphone-based systems, such as that disclosed in WO95/00946). Thepresent invention relates to the latter category.

In the early 1990s, car makers were considering buildingnoise-cancellation systems into the headrests of their cars. This is asensible approach, because the cancellation spot is well-defined (i.e.,at each respective ear of the driver), and directly accessed by aloudspeaker several inches away, such that the magnitude and phase ofthe signals arriving at the ear are well-defined. A similar system wascontemplated for aircraft.

In general, low frequency noises are easier to cancel than highfrequency ones, because the wavelengths are longer and hence theamplitude of the wave does not change as much per unit distance as itdoes for short wavelengths.

More recently, Sony have introduced noise-cancelling headphones underthe WARP trademark in Japan. WARP stands for Wave Adaptive ReductionPrinciple. These are available in two forms: (a) conventionalcircumaural style, where the driver unit sits on a padded collar whichlies around the edge of the pinna, and (b) in-ear style. More recently,Sony have also introduced a “pad-on-ear” type headphone (model MDR-NC5).

Other, similar products are made by, for example, Sennheiser, NCT, andKoss. Sennheiser manufacture several types of active, noise-cancellingheadphones, such as their HDC 451, and NCT (of Stamford, USA) have a“Noisebuster” headphone system.

FIG. 1 shows a known means of acoustic noise-reduction, comprising aheadphone driver unit (one of a pair), mounted in a headphone shell,together with an outer, noise-pickup microphone. The microphoneregisters the background noise on the outer side of the headphone shellassembly, and feeds an inverting amplifier. The output from thisinverting amplifier is added to (i.e., summed with) the music signals atthe headphone driver stage, such that the resultant noise-related signalon the inside of the headphone shell is equal and opposite to that ofthe background noise which permeates directly through and around theheadphone shell.

SUMMARY OF THE INVENTION

In order to allow for the variations in sound transmission through andaround the shell (owing to the fit on the listener's head, ears andother factors), it is useful to provide a variable amount of thenoise-cancellation signal such that the user can control and optimisethe performance of the noise-reduction by adjusting a potentiometer orsimilar device. This method assumes that the microphone is close to thenoise-cancelling acoustic driver unit (the headphone actuator). If themicrophone were to be located elsewhere, away from the headphone shell,then the noise-cancelling signal would be different from thenoise-to-be-cancelled in the shell, and so the effectiveness would bedegraded. In fact, in the extreme, the noise level would be increased.

According to a first aspect of the present invention there is provided amethod as specified in claims 1–5. According to a second aspect of theinvention there is provided apparatus as claimed in claims 6–9.According to a third aspect of the invention there is provided a systemfor programming the transfer function of a programmable filter in anoise reduction system as claimed in claim 10.

The present invention is an improved method of providing ambientacoustic noise-reduction in telephones (such as for example cordless ormobile or cellular phones, or conventional phones) and headphones, suchthat the listener can perceive the transmitted sounds more effectivelyin a noisy environment. It is based on the recognition that both theincoming noise signals, and the acoustic signal produced by the phonetransducer, are acoustically modified by the proximity of the head andears to the loudspeaker or earphone. The invention provides a method forcompensating for these effects.

A further aspect of the invention provides means to “customise” thecompensating parameters, so as to match optimally the individualphysical and/or physiological characteristics of a given listener.

It has been assumed in the prior art that the noise signal outside theheadphone shell and the broadcast signal inside the headphone shell arelinearly related, that is, they are related purely by a gain factor. Itis shown below that this assumption is not valid.

It also seems to be common practise for manufacturers to “roll-off” thehigh frequency content of the noise-cancellation signal, limiting thecancellation range of frequencies to below 1 kHz. This can be seen intheir specification sheets. It is often the lower frequencies which aretroublesome, firstly, because this is the nature of much industrialnoise (in an aircraft cabin, for example), and secondly, because LFnoise permeates and penetrates structures more than high-frequency (HF)noise, because it diffracts readily around body-sized artifacts and doesnot reflect or scatter so readily).

Although prior-art noise-cancelling headphones can be fairly effectiveat sub-1 kHz frequencies, they are nevertheless far from efficient,because much of the irritating, ambient noise lies in the range 500 Hzto 5 kHz. The present invention can mitigate this problem, and affordsthe means to reduce or cancel noise frequencies up to 5 kHz or more.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art acoustic noise reduction system,

FIG. 2 shows the effect of head proximity on a headphone mountedmicrophone,

FIG. 3 shows an ear transfer characteristic for two differentsituations,

FIG. 4 shows a typical characteristic of a miniature electretmicrophone,

FIG. 5 shows the transfer functions associated with various parts of aheadphone system,

FIG. 6 shows the transfer characteristics measured for a pad on ear typephone for direct electrical signals and leakage of ambient noisesignals,

FIG. 7 shows a filter transfer function computed to compensate for thefeatures 20 shown in FIG. 6,

FIG. 8 shows a block diagram of a noise cancellation system according tothe present invention, and

FIG. 9 shows a system for programming the transfer function of aprogrammable filter in a noise reduction system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The acoustic modifications that the incoming noise signals (and thesignals transmitted by the phone transducer) are subject to, will firstbe described. As far as the inventor is aware, neither of thesephenomena have been considered before in the context ofnoise-cancellation.

It is known in headphone noise-cancellation arrangements to situate aminiature microphone (3), such as for example an electret microphone, onthe outer section of each of the left and right headphone shell units(FIG. 1), and this provides the noise cancellation reference signal.(Microphones of this type are essentially omni-directional.) Themicrophone is typically spaced about 5 cm from the side of the head. Asa consequence, the microphone is effectively subjected to two soundsources: firstly, the incoming ambient noise; and secondly, aback-reflected wave from the side of the head. These two waves undergoconstructive and destructive wave addition. Consequently, the first (andlowest) destructive frequency at which this occurs is when the waves areexactly out of phase, when their relative displacement corresponds toprecisely one-half of a wavelength. So for the example here, where themicrophone-to-head distance is 5 cm, the overall path displacement istwice this value (i.e., 10 cm). Destructive interference thereforeoccurs when the wavelength is 20 cm, corresponding to a frequency ofaround 1.7 kHz.

In this example it has been assumed that the noise source is normal tothe lateral axis through the listener's head (and ears). In practice,the ambient noise is more likely to be present as a “diffuse”sound-field (with equal energy in all directions), and arriving from alldirections, not just on-axis. Although this reduces the comb-filteringeffect, it is nevertheless still present. The effects are shown in thespectral plot of FIG. 2, which were measured using an electretmicrophone mounted according to FIG. 1, and with the sound-source atnormal incidence. Note the first interference trough just below 2 kHz,and a second at about 4 kHz. The first interference peak can be clearlyseen at around 3 kHz.

When a cell-phone or headphone driver unit (1) is held or mountedadjacent to the outer ear (2), an acoustic cavity is formed. Thiscomprises the major outer-ear cavity (the “concha”), which is partiallybounded by the hard, reflective surface of the phone itself. This isespecially applicable to “pad-on-ear” type phones (circumaural typesgenerally tend to dampen the resonance somewhat).

The consequence of this is that (a) the headphone driver is coupledto—and driving into—a resonant cavity, and (b) the ambient acousticnoise is leaking into the same resonant cavity. The effects are notnecessarily the same because the acoustic impedances of the two pathwaysare very different. Because of its physical dimensions, the cavity willresonate most strongly at several kHz, and hence the incoming noisesignal will effectively be boosted at this frequency.

However, the resonant properties are dependent entirely on the acousticattributes of the cavity, and so differing ear-sizes create differingresonant frequencies and “Q”-factors, as does (to a lesser extent) thenature and proximity of the phone surface which bounds the concha.

In order to compensate for these acoustic phenomena, an appropriatefiltering system is created, as follows. The filtering can be carriedout either in the analogue domain, using operational amplifiers andknown filter configurations, or preferentially in the digital domainusing FIR or IIR filters (4). This latter enables better control andallows user-reconfiguration.

FIG. 3 shows two Far Transfer Characteristics (ETCs), which weremeasured using a B&K type 5930 artificial head. First, the free-fieldcharacteristics of one of the ears was measured using an impulse methodin which a small loudspeaker was used as the sound-source, placedon-axis to the right ear of the B&K head, at 0.6 m distance so as tosubstantially avoid standing-wave generation. This is essentially the“near-ear” response of the 90° HRTF (Head-Related Transfer Function).Next, a cell-phone (Ericsson type A-1018S) was held to the ear of theartificial head, in just the position that it would be if a user werelistening to it. The measurement was repeated, and yielded the secondset of characteristics shown in FIG. 3. Note that both of the FIG. 3plots have been corrected for the loudspeaker coloration, and that theB&K Head microphone coloration is negligible. The effect of partiallyoccluding the concha is clear to see. The cell-phone has disturbed theprimary concha resonance, at about 5.5 kHz, creating a new resonant peakat about 2.4 kHz, and there is also some masking of the higherfrequencies between 4 kHz. and 11 kHz.

A low-cost, miniature, electret microphone, as would be used fornoise-cancellation, was characterized against a B&K type 4006 studioreference microphone (which has a very smooth, essentially flatfrequency-response between 20 Hz and 20 kHz). This indicates the amountof spectral “coloration” that these microphones would introduce into thesignal-processing chain. The measurements were made in a quietenvironment using an impulse method. The spectral characteristics of themicrophone are shown in FIG. 4 (note that the coloration of theloudspeaker used for these measurements has also been subtracted). Themicrophone exhibits a relatively “flat” response for such a low costitem.

FIG. 5 shows the relevant transfer functions associated with the variousacoustic and electrical sip al pathways into the headphone shell.Function L represents the overall acoustic pathway from an externalsound-field through the shell bulk and also via the parallel leakagearound the edges of the cushion. For a mobile phone, where the handsetis only loosely in contact with the pinna, this latter is thepredominant of the two. Function M represents the spectralcharacteristics of the external microphone which is used to pick-up theexternal ambient noise signal, including the associated acousticmodification (comb-filtering) by the closeness of the head (refer toprevious text). Function D represents the overall transfer function intothe outer-ear cavity via the electrical transducer drive-unit, andfunction F represents a separate signal-processing is filter which is tobe used serially with D in order to accomplish optimal noisecancellation. It is now possible to define the various signal pathwaysand calculate what the transfer function F will be for optimalcancellation.

First, if the external, ambient, noise sound-field is denoted by:{NOISE}, and the electrical input to the headphone driver {INPUT}, thenthe ambient noise present in the cavity is:Noise in cavity={NOISE}*[L]  (1)

Next, the electrical delivery path into the cavity via the driver can bedefined to be:Electrical delivery={INPUT}*[D]*[F]  (2)

And, of course, the noise pick-up microphone has the transfer function:Microphone=[M]  (3)

For effective noise cancellation, we define that the serialnoise-measurement and electrical delivery paths must deliver a signalwhich is equal and opposite to the noise signal in the cavity, thus:{NOISE}*[M]*[D]*[F] must be equal & opposite to {NOISE}*[L]  (4)i.e.{NOISE}*[M]*[D]*[F]=−1*{NOISE}*[L]  (5)

and therefore

$\begin{matrix}{\lbrack F\rbrack = \left\{ {\frac{{- {NOISE}}*\lbrack L\rbrack}{{NOISE}*\lbrack M\rbrack*\lbrack D\rbrack} = \left\{ \frac{- \lbrack L\rbrack}{\lbrack M\rbrack*\lbrack D\rbrack} \right\}} \right.} & (6)\end{matrix}$

This has now defined the filter function [F], required to process theincoming noise signal so as to create optimal cancellation in theheadphone cavity.

Some of the above transfer functions have been shown in earlier Figures.Typical microphone characteristics, M, including the head-proximitycomb-filtering effects, are depicted in FIG. 2, and the ambient leakagecharacteristics, L, of a cell-phone are shown in FIG. 3 (“cell-phonepresent”). Clearly, the required filtering F is dependent on severalphysical factors, and will vary according to their dimensions andconfiguration. In order to depict the nature of a typical filter factorF, the following example uses data relating to a pad-on-ear headphone(which is similar to a cell-phone in terms of outer-ear interaction, andit is conveniently accessed electrically).

FIG. 6 shows data from measurements made using a pair of mid-price,pad-on-ear type headphones mounted on a B&K head. The acoustic leakagewas obtained using the same method as described earlier for thecell-phone, and the electro-acoustic transfer function was obtained bydriving an impulse directly into the phone, and measuring the signalfrom the relevant artificial head microphone. These particular phonecharacteristics are relatively flat, which is quite unusual.

FIG. 7 shows a filter characteristic [F] according to equation 6, basedon the data of FIGS. 6 and 2. Note that the frequency scale has beenexpanded to show only 0 to 10 kHz, as this is practical region ofoperation, bearing in mind the day-to-day physical differences which canoccur when placing the phone against the ear, and the present bandwidthlimitations of telephony (<4 kHz). The data plotted in FIG. 7 haveaccrued some noise because of the combined number of sources used forits calculation, but it can be seen that there are several mainfeatures: (a) a 10 dB fall between 0 and 600 Hz; (b) a flat regionbetween 600 Hz and 2 kHz; and (c) a broad trough between 2 kHz and 4kHz. However, there is little significance, if any, in this particularshape: it is merely the combination of the factors of its constituentcomponents.

It will be appreciated that the filtering [F] will need to match notonly the to amplitude characteristics of the entrained ambient noise[L], but that is also has corresponding phase characteristics. This canbe achieved using either FIR filters or IIR filters.

As the outer-ear is one of the primary influencing factors in theinvention, the physically-related parameters can be “tuned” so as tomatch each individual's ears. This applies to (a) the leakagecharacteristics, and especially to the size and position of the resonantpeak (part FIGS. 3 and 6); and (b) the electro-acoustic phone-to-eartransfer characteristic (FIG. 6).

This enables the application of the “Virtual Ear” tailoring-typeapproach to the modification of filter parameters, as described in ourco-pending patent application number EP 1 058 481, which is incorporatedherein by reference. Either a series of “set-up” tests, or,alternatively, a range of pre-set values can be provided for the user toswitch through and select the optimal. (The latter is probably moreconvenient.) A computer located remote from the phone or processingmeans present in the phone could, for example, carry out (and optimize)the set-up, and then program the parameters for the individual user. Thefinal compensation filter [F] will also be dependent on the phonedimensions and structure, and is it is likely that each different phonetype will have slightly different characteristics in this respect.However, these are trivial to measure and program. If an externalcomputer is used, it can advantageously provide via a separate soundgeneration means an ambient noise signal (such as white or pink noise)in which a broad band of frequencies is present, in order to aid fulloptimization.

FIG. 8 shows a block diagram of a telephone, in the present example acell-phone. The incoming, ambient noise is picked up by the outermicrophone on the cell-phone handset, and then it is processed bycompensation filter F (which includes the inversion factor). Next, it ispassed through a gain-adjust stage, such that the user can fine-tune thecancellation level, passed to a summing drive amplifier, and thence tothe internal phone loudspeaker. The phone's internal audio source (i.e.,from the radio receiver) is also fed to the summing drive amplifier. Amicro-controller integrated-circuit is used to control the parameters ofthe compensation filter, F, such that they can be adjusted to suit theindividual's outer-ear characteristics, as has been described. Themicro-controller, in turn, is connected to the keypad of the telephone,such that the user can make the necessary adjustments (including thegain adjust, above).

FIG. 9 shows a block diagram of a method of calibrating the system to anindividual user by means of a personal computer (PC). The system issubstantially as shown in FIG. 8, with the addition of a PC which isdigitally connected to the phone's micro-controller, either directly viaa serial or parallel cable, or indirectly via radio or optical means.The user listens to a test signal from the phone loudspeaker,representative of a distant caller, whilst the computer creates ambientnoise signal and feeds this to one of its loudspeakers. By adjusting thefilter parameters, or, alternatively, selecting from a number ofavailable pre-set filter functions, the user can set-up the optimalsignal processing configuration for their own personal physiologicalcharacteristics. The use of a PC is convenient because it allows muchmore precise control of adjustment via its mouse and cursor control keysthan the small phone keypad, which is also in an inconvenient positionvery close to the listener's head. Of course, the micro-controller couldbe used to perform the programming itself in a less preferredembodiment. The computer can be used to store individual preferences,and provides a more sophisticated method of interaction with thecell-phone. In both FIG. 8 and FIG. 9 the signal processing apparatus isdenoted by the reference numeral 5.

Finally, the content of the priority document, especially the drawings,is incorporated herein by reference.

1. A method of improving the audibility of sound from a loudspeakerlocated close to an ear, comprising the steps of detecting ambientacoustic noise arriving from other sound sources using a transducer thatprovides corresponding ambient sound signals; inverting the polarity ofsaid ambient sound signals and combining them with the signals being fedto the loudspeaker to reduce the audibility of said ambient acousticnoise; passing said ambient sound signals through a filter having apredetermined average transfer function that compensates for thespectral modification of sounds travelling from the loudspeaker to theear caused by the proximity of the loudspeaker to the ear of a listenerin use.
 2. The method of claim 1 wherein the transfer function alsocompensates for the spectral modification of the detected ambient noisesound signal caused by the proximity of the transducer to the head of alistener in use.
 3. The method of claim 1 in which the transfer functionalso compensates for the modification of the ambient noise sound signalcaused by the proximity of the loudspeaker to the ear of a listener inuse.
 4. The method of claim 1 in which the transfer function is one of aplurality of transfer functions which are user-selectable in use.
 5. Themethod of claim 1 wherein the filter is an FIR or IIR filter.
 6. Anapparatus for improving the audibility of sound from a loudspeakerlocated close to an ear, comprising: a headphone system; acoustic noisereduction means comprising a filter, the filter having a predeterminedtransfer characteristic that compensates for the spectral modificationof sounds travelling from the headphone to the ear caused by theproximity of the headphone to the ear of a listener in use.
 7. Theapparatus of claim 6 having a ambient acoustic noise reduction meansincluding a transducer for detecting ambient acoustic noise, and afilter having a predetermined transfer characteristic which compensatesfor the spectral modification of the detected ambient acoustic noisecaused by the proximity of the transducer to the head of a listener inuse.
 8. The apparatus of claim 6 in which the filter also compensatesfor the modification of the ambient noise sound signal caused by theproximity of the loudspeaker to the ear of a listener in use.
 9. Theapparatus of claim 6 wherein said headphone system is a telephone.
 10. Asystem for programming the transfer function of a programmable filter ina noise reduction system for improving the audibility of sound from aloudspeaker located close to an ear comprising: a transducer locatedclose to said loudspeaker for detecting ambient sound signals; andsignal processing means comprising: means to invert the polarity of saidambient sound signals and combine them with the signals being fed to theloudspeaker; a sound source for generating ambient sounds over a rangeof frequencies, the sound source being remote from the loudspeaker; andcontrol means for providing a plurality of user-selectable transferfunctions which can be programmed into the programmable filter such thata user can select the optimum transfer function in use wherein theuser-selectable transfer functions provide compensation for at leastspectral modification of sounds travelling from the loudspeaker to theear caused by the proximity of the loudspeaker to the ear of a listener.11. The system as recited in claim 10 wherein the user-selectabletransfer functions further provide compensation for modification of theambient noise sound signal caused by the proximity of the loudspeaker tothe ear of a listener in use.